Recently, detergents or chemicals such as agrochemicals are hermetically-packaged (unit-packaged) with a water-soluble film in certain amounts and the packaged products are directly put into water for the convenience of the consumer and the safe use by the consumer. As the water-soluble film dissolves in water, the contents are released. Thus, the effects of the contents are obtained. Such a technique is quite useful in the packaging field because it enables the use of the contents without direct touching and eliminates the measure of the amount of the contents and disposal of the package films.
Water-soluble films are typically made of a partially saponified polyvinyl alcohol-based polymer (also referred to as PVA) in terms of water solubility and strength. However, films made of PVA are poor in properties such as solubility in cold water. In order to solve such a problem, a technique of partial anionic modification of PVA or introduction of a water-soluble monomer into PVA by copolymerization has been proposed (see, for example, Patent Documents 1 to 5). In addition to these, water-soluble films made of PVA for packaging chemicals or detergents, for example, are also disclosed (see, for example, Patent documents 6 to 9).